1. Field of the Invention
The present invention relates to a data communication device for receiving a signal in the ASK (Amplitude Shift Keying) format, a vehicle-mounted communication system using thereof, and a data communication method.
2. Description of Related Art
On-vehicle electronics technology has been rapidly developed in recent years. In such a trend, TPMS (Tire Pressure Monitoring System) is applied to tires at about 60 percent currently in the US market and the application thereof is expected to reach 100 percent in the future by legislation to be in force.
Technique related to the TPMS is described in, for example, Japanese Laid-Open Patent Application JP-P2006-109274A (refer to Patent Document 1, corresponding to US2006/079190 (A1)).
The TPMS usually uses a LF (low frequency) radio wave such as 125 KHz as a carrier wave which is ASK-modulated by using a digital baseband signal. A signal (hereinafter referred to as transmission radio wave) ASK-modulated is received by a resonance antenna (i.e. antenna resonance circuit) and demodulated by a detection circuit in a data communication device used in the TPMS.
The data communication device determines data “0” and data “1” by analyzing a digital baseband signal demodulated by an arithmetic processing device. If the Manchester encoding method is employed here as a data determining method, the arithmetic processing device determines data “0” in the case where a signal level of a digital baseband signal is in transition from a high level to a low level within one period, and determines data “1” in the case where it is in transition from a low level to a high level within one period.
A technique related to a receiving device which uses Manchester encoded data is disclosed in, for example, Japanese Laid-Open Patent Application JP-P2005-142615A (refer to Patent Document 2).
In order to extract “0” and “1” with high accuracy from a transmission radio wave which is ASK-modulated, it is necessary to demodulate the transmission radio wave into a proper digital baseband signal. However, a data reception error may occasionally occur because a desired digital baseband signal cannot be obtained due to a fluctuation of reception intensity based on a propagation environment such as noise and a voltage fluctuation on a transmission side and/or a reception side.
Moreover, even if a state that a transmission radio wave exists is changed to a state that a transmission radio wave does not exists, when large electric charges are accumulated in the antenna resonance circuit, it takes prolonged time to attenuate the reception intensity in a low level. In this case, if a residual voltage in the antenna resonance circuit is larger than a threshold value, data may be occasionally determined as “1”. That is, residual electric charges in the resonance circuit cause reception of “1” even though transmitted data is “0”, thereby causing the TPMS to malfunction. It is therefore desired to provide a technique which allows discharge of energy accumulated in the resonance circuit.
For example, a radio wave receiving device for preventing occurrence of a reception error due to noise is disclosed in Japanese Laid-Open Patent Application JP-P2005-223478A (refer to Patent Document 3, corresponding to U.S. Pat. No. 7,369,831 (B2)). The radio wave receiving device according to Patent Document 3 is provided with an antenna short-circuit control means adapted to periodically short-circuit a resonance antenna at predetermined time intervals in order to discharge energy accumulated by a reception signal. Energy accumulated in the resonance antenna can be therefore discharged periodically, so that an attenuation time to attenuate energy accumulated in the resonance antenna can be shortened.
However, we have now discovered the following facts.
In the Patent Document 3, timing to short-circuit the resonance antenna is not determined by taking into consideration a time range of level determination for a reception radio signal. The resonance antenna is also short-circuited several times within a period (here 1 S) of a modulation wave. In this case, signal intensity is decreased after passing through a filter as described in Patent Document 3.
Meanwhile, the level determination is carried out at predetermined timing in the case of extracting data “0” and “1” from a signal which is ASK-modulated. Here, if the resonance circuit is short-circuited in the time range of the level determination as taught by the technique according to the Patent Document 3, it is impossible to obtain desired amplitude and there is a danger that a reception error may occur. For example, “0” may be occasionally determined even if a signal corresponding to data “1” is supplied, because a signal level does not exceed a threshold value due to discharge resulting from short-circuit.